A modification of this type of derailleur has been proposed in U.S. Pat. No. 3,974,707 in which the derailleur has a control member having a plurality of recesses corresponding to the number of speed change stages mounted on one of the four members and a ball engageable with each of the recesses carried by another member in relative motion to the one member, so that when both members relatively move, the ball travels with respect to the recesses and engages therewith to be positioned and retained in accordance with a desired speed change stage.
This construction makes it impossible to return a chain guide by action of a return spring, because the ball engages elastically with each of the recesses to retain the chain guide in position, which results in the need for use of two cables or the so-called push-pull type cable in order to forcibly reciprocate the chain guide.
When no return spring is used, the ball must be subjected to a sufficient force for disengaging it from each of the recesses, whereby a control lever is lightly operable. Unfortunately, the control wire cannot be formed by the usual pull type wire making it expensive to produce.
Another modification of this type of derailleur has been proposed in U.S. Pat. No. 4,030,375 in which the derailleur has a chain guide which is moved in one direction by pulling the pull-type wire and returned in the opposite direction by action of the return spring, and has at one of the four derailleur members, a cam having a plurality of notches corresponding to the number of speed change stages and at another member in relative motion to the one member, a roller or pin engageable with the notches, so that the roller or pin engages with a selected one notch, thereby accurately positioning the chain guide in accordance with a desired speed change stage.
With this construction it is possible to position the chain guide in accordance with a desired speed change stage even when the control cable changes in length. However, one problem with this arrangement is that it is structually impossible to maintain the chain guide in position through the notches, thereby requiring the application of frictional resistance to a control lever for overcoming the strength of the return spring, which results in the need for a large force to operate a control lever. Another problem is that this type of derailleur produces noise when a driving chain is switched to a selected sprocket. In explanation of this problem, the roller or pin moves along the cam face with respect to the notches following the relative motion between two of the four derailleur members by a pull of the control wire or action of the return spring. The path of movement of the roller or pin across adjacent notches describes a circular arc around the border therebetween. As a result, the chain guide, when the chain is switched to a selected sprocket by a pull of the wire, moves following the operation of the control wire without intermittence. Accordingly, when the wire is operated to reciprocate the chain guide, the chain slowly contacts with a sprocket to which the chain is to be switched thereby generating noise and increasing wear of the chain and sprocket.
To solve the aforesaid problems, a derailleur has been proposed which is provided with means through which a retainer engaging with one of the notches is forcibly moved to disengage therefrom, so that the return spring acts to quickly move the movable member. This, however, is effective only for quickly moving the movable member by action of the return spring. Hence, when a driver slowly operates the lever to pull the control cable, noise is still generated and wear of the chain and sprocket still occurs.